1. Field of the Invention
The present invention relates to a thin film transistor substrate, a manufacturing method of a thin film transistor, and a display device using the same.
2. Description of Related Art
An active matrix liquid crystal display device which uses a thin film transistor (TFT) substrate is widely used, as described in Japanese Unexamined Patent Application Publication Nos. 2001-217423 (Hayashi), 2002-26330 (Nakamura), and H10-178177 (Takeguchi). A TFT substrate of a related art is described hereinafter with reference to FIG. 3. FIG. 3 is a sectional view showing the sectional structure of a top-gate TFT substrate according to a related art. In the TFT substrate 101 in FIG. 3, a SiN (Silicon Nitride) film 12, which serves as an anti-diffusion layer, is deposited on a glass substrate 11. On the SiN film 12, a semiconductor layer 13, which includes a channel region 131, a source region 132 and a drain region 133, and a lower capacitor electrode 13a, which is made of the same material as the semiconductor layer 13, are formed in island shapes. The lower capacitor electrode 13a serves as an auxiliary capacitor region of a TFT 22.
Further, a SiO2 (Silicon Dioxide) film 14, which serves as a gate insulating layer, is deposited to cover the semiconductor layer 13 and the lower capacitor electrode 13a. An island-shaped gate electrode 15 is then formed above the channel region 131 with the SiO2 film 14 placed therebetween. Specifically, the SiO2 film 14 is placed between the gate electrode 15 and the channel region 131, so that the gate electrode 15 is placed opposite to the channel region 131 of the semiconductor layer 13 with the SiO2 film 14 interposed therebetween. The channel region 131 of the semiconductor layer 13 and the gate electrode 15 are thus placed opposite to each other with the SiO2 film 14 interposed therebetween. Further, an island-shaped upper capacitor electrode 15a is formed above the lower capacitor electrode 13a with the SiO2 film 14 placed therebetween. The upper capacitor electrode 15a is made of the same material as the gate electrode 15.
Then, a SiO2 film 16, which serves as an interlayer insulating layer, is deposited on the gate electrode 15. The source region 132, the drain region 133, and the lower capacitor electrode 13a are respectively connected with a line electrode 17 through contact holes 20 which are formed in the SiO2 film 14 and the SiO2 film 16. The line electrode 17 is connected with the source region 132 to supply a display voltage to the TFT 22. The line electrode 17 is also connected with the drain region 133 to form a given circuit, such as supplying a display voltage to a pixel electrode.
Further, an upper insulating layer 18 is deposited on the line electrode 17. The line electrode 17 is connected with a pixel electrode 19 through a contact hole 21 which is formed in the upper insulating layer 18. The line electrode 17 is electrically connected with the source region 132 and the drain region 133 to transmit a picture signal or a control signal from the outside or the inside of a circuit on the substrate. As described above, in the top-gate TFT substrate of a related art, the line electrode 17 for connecting a plurality of elements such as a TFT and a capacitor is placed in an upper layer of the TFT substrate 101.
However, the present inventor has recognized the followings. In order to establish a connection of the line electrode 17 with the source region 132, the drain region 133 and with the lower capacitor electrode 13a to supply a display voltage to the pixel electrode 19, it is necessary to form the contact holes 20 and 21. The manufacturing process thus requires two steps of contact hole formation. Specifically, the process needs to form the contact holes 20 between the line electrode 17 and the source region 132, between the line electrode 17 and the drain region 133, and between the line electrode 17 and the lower capacitor electrode 13a, and further form the contact hole 21 between the line electrode 17 and the pixel electrode 19. Therefore, the number of masks and manufacturing steps which is required for forming contact holes is two, which is an disadvantage in manufacturing cost reduction.
In view of the foregoing, it is an object of the present invention to provide a thin film transistor substrate, a manufacturing method of a thin film transistor, and a display device which require a smaller number of masks and manufacturing steps by integrating contact hole formation steps.